I had D1, DX and D2. I would get D2X since the price diffrence is not that big. You get better and better sound as you go up the food chain but i would stick with D2X for the moment. There was a Xonar I always wanted because it had a built-in headphones amplifier, I forgot its name. It goes for about 120£, worth checking it out. But if youre low on budget go for DX, pretty decent for the moeny.
Changing the sound card is not a big deal. Before buying the card make sure you have that particular slot free on your motherboard. PCI-Express1X is the little tinny one, PCI is the long white one and PCI-Express is the long colourfull one. After fitting the card in boot up your PC and go into BIOS (press Del or F2 or F10 or F12 , depending on your motherboard) and look around in that menu to turn off integrated soundcard. In Windows you just put your soundcard CD in and install whatever pops up. I would also go on their official web site for the latest drivers updates.
Asus vs Creative: yes, Creative have a good name but theyve been sleeping on it for too long. Asus are more value for money and they dont crash every other minute.
To take advantage of a good soundcard you will need to cut down on bad habbits like mp3, divX movies, cheap headsets etc. Hope this helps